Sunken Treasure, Mint Condition

When do coins from a shipwreck get corroded?

On Friday, a team of deep-sea treasure hunters announced the discovery of a shipwreck containing 17 tons of Colonial-era coins worth as much as $500 million. The team's sponsors haven't revealed where in the Atlantic they recovered the cache or what ship carried all of that loot. They did, however, note that the gold and silver coins were in pretty good condition. What factors affect the quality of shipwreck coins?

Torie Bosch is the editor of Future Tense, a project of Slate, New America, and Arizona State that looks at the implications of new technologies.

Where the ship goes down and what kind of metal the coins are made of. Coins that spend hundreds of years submerged can end up getting scratched, worn down, corroded, covered by sea life or lime deposits, or damaged by acid conditions. The warm waters of the Caribbean and the tropics are likely to cause the most damage, as warmer temperatures speed up oxidation and corrosion. These waters also contain coral and micro-organisms that can encrust the coins, depleting their value, usually permanently. Cooler northern seas—like those off the coast of England, where some speculate this treasure was uncovered—are more likely to help keep all kinds of coins looking good.

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Conditions at the sea floor can also make a difference. A muddy bottom might help preserve coins by encasing and protecting them, but an environment of swirling sand can cause scratches and wear down markings and designs. (The depth of the wreck also comes into play: Deep waters tend to have weaker currents, so the sand at the bottom doesn't move around as much.) In some cases, though, sand can be a good thing. The S.S. Central America sank in 1857 amid calcium carbonate sands that helped make the surrounding water slightly alkaline, keeping potentially damaging acidity at bay. As a result, the ship's coins were close to pristine when they were uncovered in 1987.

Salt water can seriously damage silver and copper coins pretty quickly, but it has almost no effect on gold. But even a gold coin can suffer damage if the ship's wood breaks down and makes the local environment more acidic.

Bonus Explainer: Why did these ships carry so much gold? For security, in many cases. Many of the wealth-laden ships that sank in the Caribbean and the Florida Straits were part of "treasure fleets" operated by the Spanish to protect loads of gold en route from the New World to Europe. The Spanish placed large quantities of treasure under the protection of these packs of ships, which were more likely to scare off pirates.

Explainer thanks John Albaneseof the Numismatic Consumer Alliance,Bob Evans and Douglas Mudd of the American Numismatic Association, and Scott A. Travers, author of The Coin Collector's Survival Manual.